1. Field Of The Invention
The invention relates to a device for detecting environmental radon gas concentrations. In particular, the invention is directed to a simple, low cost device for measuring the actual exposure of a person to radon gas, as well as that person's potential exposure due to his or her remaining in any single location substantially all the time.
2. Description Of The Prior Art
Radon gas (.sup.222 Rn) is a decay product of the element radium (.sup.226 Ra) which is found in soils and rocks throughout the United States and the world. Radon can diffuse through cracks in rocks and through soil pores and enter the breathable atmosphere. Structures such as homes and other buildings can "trap" the .sup.222 Rn inside them because of typical low air ventilation rates. Concentrations of radon gas can possibly rise to high levels. High concentration levels and their adverse effects on humans are known.
Although average household levels are probably orders of magnitude less than mine levels, the risk at these lower concentrations for developing lung cancer may not necessarily be insubstantial, due to the long time that people spend indoors and sometimes spend living in the same home.
With the threat of lung cancer definitively linked to exposure to .sup.222 Rn, it has become necessary to determine where levels of the radon gas are acceptable and where they represent a significant health risk. This can only be done through actual measurement. Stationary measurements, while useful, may not truly represent actual exposure levels of individuals, because people rarely spend all of their time in any single location.
Accordingly, currently of substantial scientific interest is the issue of actual personal exposure as compared to exposure levels in given locations. Because people tend to move about from place to place, and the time spent in any single location such as in the home or workplace may vary from person to person, there has arisen a need for a device which could conveniently serve as a stationary monitor or a personal radon gas monitor.
In general, continuous radon detection instrumentation is costly, large and requires electric power. Such detection equipment is also known to often require operation by highly skilled technicians, and is generally used for research purposes.
In response to the need for large-scale stationary monitoring of radon gas levels, an "integrating" monitor, in which a signal from the varying concentration of radon gas or its products is accumulated and then averaged over the exposure period to obtain a mean concentration, was developed and described in Harley et al., U.S. Pat. No. 4,800,272, Environmental Gamma-Ray And Radon Detector. The entire disclosure of U.S. Pat. No. 4,800,272 is hereby incorporated by reference.
The detector of U.S. Pat. No. 4,800,272 employs lithium fluoride (LiF) thermoluminescent dosimeters (TLDs) to measure both radon gas exposure and gamma-ray exposure. TLDs are known to be reliable radiation measurement devices. Incident radiation displaces valence electrons in its atomic structure. These electrons are trapped in crystal defects intentionally introduced into the crystal lattice structure, and are released under application of sufficient heat energy. Electron release is accompanied by the emission of light in the 250-400 micron wavelength band. The number of photons emitted per unit time period (under predetermined readout conditions) is directly proportional to the radiation exposure, and the analyzing process is entirely electronic (i.e., no optical counting is required).
In the detector of U.S. Pat. No. 4,800,272, three TLDs serving as radiation detectors are enclosed in a small electrically conductive (shielded) housing. A first one of the TLDs is covered by a protective metallized MYLAR (polyester film) sheet, and detects environmental gamma radiation. A second one of the TLDs is covered by an electrostatically charged dielectric materials, or "electret," to concentrate and collect positively charged radon daughters. Further decay of the so-collected radon daughters produces alpha particle damage to the TLD which may later be measure as noted above. A third one of the TLDs is predosed with gamma radiation in order to provide a reference signal indicative of an amount of fading of the first TLD signal representing gamma-ray exposure.
The passive device of U.S. Pat. No. 4,800,272 is intended to remain in a single location for several months, and thus short-term sensitivity is not a concern. Also, the detection limit (also called sensitivity, or lower limit of detection) of the detector has been found to be approximately 90 picoCuries per liter-days (pCi/L days), which is unsuitable for shorter-term measurements of extremely low exposure levels. Although compact, the size of the device of U.S. Pat. No. 4,800,272 is still nonetheless unsuitable for personal radon exposure monitoring.
In an effort to increase the sensitivity (i.e., reduce the lower limit of detection) and reduce the size of a passive gamma-ray and radon monitor, it was attempted to use more sensitive detector materials in the monitor of U.S. Pat. No. 4,800,272. In particular, another known integrating radon detecting material is the solid state nuclear track detector (SSNTD). An SSNTD film such as cellulose nitrate, cellulose acetate, or a carbonic acid diester, incurs radiation damage tracks left by alpha particles emitted by daughters RaA and RaC'. A very small percentage of alpha radiation directly from radon gas decay into its daughters is also detected by the SSNTD. The alpha particles penetrate the dielectric material, leaving tracks which can be made optically visible by chemical etching for subsequent counting.
U.S. Pat. No. 4,417,142 to Malmqvist et al. describes such an alpha detector which includes a cellulose nitrate film. Malmqvist et al. further describe the desirability of preventing the build-up of static electricity on the detector. To do so, Malmqvist et al. describe covering the cellulose nitrate film with an electrically conductive layer.
While advantageously passive in operation, inexpensive and small in size, generally the SSNTD has not in the past been regarded as highly accurate, due to difficulties in track counting. At low environmental levels, track counting has been especially difficult due to presence of flaws in the detector material which appear as tracks, variations caused by the handling and storage history of the particular detector, and variability in the etching process for enhancing track visibility.
Furthermore, when SSNTDs are used in arrangements similar to that of U.S. Pat. No. 4,800,272 or FIG. 3 of U.S. Pat. No. 4,417,142, radon radiation alpha track counting is difficult because of non-uniformity of electric charge on the electret or conductive film. Such non-uniformity of charge has been observed as causing tracks to occur in dense clusters, rendering track counting very difficult, and sometimes impossible.
Measurement errors introduced by the particular transportation, handling and storage history of each given SSNTD have proven to be especially troublesome. In particular, the detector material begins to register tracks from the moment its alpha-impermeable protective film is removed during assembly of a radon monitor. For large-scale monitoring programs, such monitors may spend a considerable period of time, after assembly, in warehouse storage, in transit to or from the testing site, and/or in temporary storage at intermediate distribution and collection facilities. Radon gas levels at each of these locations and the time spent by the device at each such location are factors which are generally impossible to determine or control to any degree of accuracy. Thus, even the most technologically advanced and accurate detector material may be useless in practice.
Finally, an activated carbon monitor which uses a sealed canister filled with activated carbon is known for short-term radon gas exposure monitoring. When the canister is opened, radon gas is adsorbed onto the carbon; the canister is then resealed after a relatively brief period of time, for example, three days. This pre-measurement sealing has long been understood as a necessity because the activated carbon monitor has a useful life after exposure for a period of only about one week. The gamma-ray emissions from radon daughters trapped on the activated carbon may then be counted. This device is highly sensitive to humidity changes and also has the drawback of variable accuracy, since it is possible for radon to desorb from the carbon during the sampling period.
Thus it is seen that there is a need for a compact, silent, low-cost and portable radon detector which has a very low detection limit (i.e., is highly sensitive), and which can be used for stationary measurements or worn on a person's body, so that actual radon exposure levels can be monitored and compared with simultaneously measured environmental radon levels. Moreover, there is need for such a device whose monitoring accuracy is reliably unaffected by the storage and handling history of that device. None of the aforementioned devices have proven satisfactory for this purpose due to the various reasons mentioned. Thus, it has not yet been feasible to undertake highly accurate large-scale radon measurement programs to determine the actual effects of different environmental levels of radon exposure in private residences and commercial buildings on a particular person or particular group of people, where the sites or persons whose exposure levels are being monitored are remote from the radon monitor assembly and laboratory testing facilities.